The Corrections Department has failed to update the Ombudsman's prison inspection team about a prisoner who was tied to his bed.
The prisoner was tied to his bed in February after three incidents of self-harm, according to Corrections.
The Department described him as a complex prisoner who was a danger to himself and staff, but said he was no longer being tied to his bed.
The Ombudsman raised concerns in May about the length of time the prisoner was tied down and the Corrections' chief inspector launched an investigation, promising to provide the Ombudsman weekly updates.
However, chief Ombudsman Judge Peter Bouchier has told RNZ that Corrections took three weeks to release its first and only weekly update.
When the Ombudsman queried the progress of the investigation on 23 June, Corrections said it would be completed by the end of the month.
But the Chief Ombudsman is yet to receive a copy of the investigation's findings.
The investigation - carried out by chief inspector Andy Fitzharris - will look at the rationale behind using the technique and all actions taken by staff.
He will also look at the relevant domestic and international law and how the case was managed.
The Ombudsman's Crimes of Torture Act inspection team has also launched an investigation.
Judge Boshier said the team was on target to complete its investigation by the end of the month.
The Ombudsman monitors prisoner treatment and conditions and has full access to prisons.
There are two inspectors who make regular prison visits and investigate allegations of ill-treatment and the use of isolation, force and restraint.
Judge Boshier told RNZ he believed resourcing for prisoners with mental health issues was inadequate and he intended to pursue the matter.
Meanwhile, Corrections has declined to answer questions about the length of time the prisoner has been tied down since February. RNZ also asked questions about how he was fed, given exercise and showered.
A spokesperson for the Department said the answers to those questions would form part of the investigation's findings.
She said the investigation was taking longer than expected, and it was not known when it would be completed.
29 February: Sentenced prisoner inside Paremoremo tied to his bed after three incidents of self-harm.
3 May: Corrections launches investigation after concerns raised by the Ombudsman. Corrections also agrees to send the Ombudsman weekly updates.
25 May: The Ombudsman receives the first and only update.
23 June: The Ombudsman's Chief Inspector of Torture emails Corrections for progress on the report. Corrections responds saying the report on track for completion by the end of the month.
9 August: The Ombudsman is yet to receive the Corrections report, however, its own investigation is due to make its findings by the end of the month.